1. Field of the Invention:
This invention relates to a clutch mechanism for a camera having a winding and rewinding mechanism and more particularly to a clutch mechanism arranged to allow or to intermit transmission of a driving force between a film take-up spool and a take-up spool driving member.
2. Description of the Prior Art:
Generally, in a camera of the kind having a sprocket arranged to feed one frame portion of film at a time by turning a predetermined degree in response to each film winding operation and a spool arranged to take up the portion of film thus payed out, the moving extent of an arbitrary point on the periphery of the spool in the turning direction thereof is arranged to be greater than that of an arbitrary point on the periphery of the sprocket in order that the coil of film on the spool is tightened without fail. Assuming that the moving degree of the arbitrary point on the periphery of the sprocket is 1, the moving degree of the point of the periphery of the spool hereinafter will be called a peripheral speed ratio. The tight winding effect on the spool increases with this peripheral speed ratio. In taking up the film on the spool, the peripheral speed ratio comes to increase and thus incessantly varies as the radius of rotation is gradually increased by the increasing thickness of the coil of film taken up on the spool. This necessitates some mechanism that is capable of constantly absorbing the peripheral speed ratio. For this purpose, a known spool friction mechanism is arranged, for example, in a manner as shown in FIG. 2 of the accompanying drawings. In this mechanism, a take-up spool 2 and a spool gear 17 are coupled by means of an embracing spring 18 with the spool gear 17 and the spring 18 arranged to be caused to slip on each other by a torque greater than a predetermined value. The peripheral speed ratio in the initial stage of a film winding process is determined by the number of teeth of the spool gear 17. Assuming that the peripheral speed ratio at a certain point of time during the film winding process is 1.5, a difference in the moving extent is arranged to be absorbed by slipping to an extent corresponding to the difference of 0.5.
However, the above-stated spool friction mechanism and the peripheral speed ratio are important factors relative to problems arising in the automation of film winding, film rewinding and film loading operations as described below:
(i) As mentioned in the foregoing, the greater the value of the peripheral speed ratio, the faster the film coil tightening process. This bears an important meaning for automatic film loading (hereinafter called AL). Before the film is automatically set at a frame number 1, a film portion approximately corresponding to four frames is idly fed in general. Then, to prevent pictures taken on actually photographed frames from being damaged by friction between the film and the wall of a spool chamber, the coil of film taken up must be completely tightened on the spool during the blank feeding. Therefore, faster film coil tightening on the spool is advantageous. It is thus desirous to have a greater peripheral speed ratio. However, this requirement contradicts the following factors:
(ii) Although the peripheral speed ratio is absorbed by the slip, the friction torque produced by the slip eventually comes to impose a load on film winding toward the end of the film winding process. The load on film winding increases with the set value of the friction torque and the extent of the slip. The slipping extent is determined by the peripheral speed ratio. The former increases accordingly as the latter increases. Therefore, in respect to the winding load, it is preferable to have a smaller peripheral speed ratio value contrary to the requirement mentioned in paragraph (i) above.
(iii) In a general camera mechanism, the spool friction becomes a load at the time of film rewinding. In the event of a film rewinding operation arranged to be carried out either manually or by means of a large output of a motor drive, etc., a load such as the load imposed by the spool friction is almost negligible for the rewinding force available and the rewinding operation usually can be carried out through the friction. However in cases where rewinding is to be automatically accomplished with a small motor disposed within the camera body, the spool friction comes to present a load which is not negligible for the available driving force. Therefore, this calls for reduction in load not only for film winding but also for film rewinding.
Of these three problems mentioned in paragraphs (i)-(iii) above, there has been proposed a method for solving the problem of paragraph (iii) by providing a clutch between the spool gear shown in FIG. 2 and a driving member arranged to drive the spool gear. This method is disclosed, for example, by U.S. Pat. No. 4,373,795. The clutch is arranged to cut off transmission or connection to permit the spool to be freely turnable for film rewinding so that the problem of Paragraph (iii) can be solved.
Meanwhile, other problems that are relative to the peripheral speed ratio as mentioned in paragraphs (i) and (ii) still remain unsolved by the provision of the above-stated clutch. In the case of the above-stated clutch arrangement, a rotation loss almost always arises in the turning direction at the time of change-over from a disconnected state to a connected state. With respect to the sprocket and the spool, the spool does not come to turn until the clutch is completely connected while the sprocket is rotating. The peripheral speed ratio thus decreases in a degree corresponding to the extent of this delay. As to the winding load, the problem of arranging the peripheral speed ratio to be small in consideration of the winding load and to be large for AL as mentioned in Paragraphs (i) and (ii), the loss of the peripheral speed ratio arising at the time of the above-stated engaging operation of the clutch presents a serious problem.
To minimize the loss of the peripheral speed ratio with a face type clutch employed, a good result of course can be obtained by reducing the pitch of the upper and lower teeth constituting the clutch. For example, if the pitch of the upper and lower teeth is arranged to be 60.degree., the maximum angle of causing the loss of the peripheral speed ratio is 60.degree. and is thus equal to the pitch. Therefore, in terms of the loss of the peripheral speed ratio, it is advantageous to make the pitch finer. However, if the pitch is excessively fine, each tooth not only comes to have a narrow width and less strength but also requires a complex machining process thus presenting another problem.